In a satellite navigation system, multiple satellites, of known positions encode their transmissions in a manner that permits a receiver to calculate its "pseudo-ranges" to the transmitting satellites. The receiver then uses the calculated pseudo-ranges from several satellites to resolve its own position. Examples of such systems are the Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS). The range measurements, and the resulting calculation of a receiver's position, vary in accuracy, due to the adverse effects of multipath and other interference on the received signals.
The accuracy of range measurements from the satellites generally decreases as the satellites approach the horizon. This is primarily due in part to the increased magnitude of reflected or refracted signals that result in multipath interference. Some current systems thus provide for rejection of signals from all satellites having an elevation below an operator-controlled cutoff. However, measurements do not consistently degrade with decreasing satellite elevation. A measurement from a satellite having five degrees elevation may be more precise, or exhibit less variability, than a measurement from a satellite having thirty degrees elevation. The accuracy of the measurements also varies according to azimuth, because of differences in the terrain features that cause multipath interference and also because of the effects of interfering signals from various other sources.